The joining together of three metal sheets is known in the prior art to require two separate welding steps and require two-sided access thereto: a resistance welding step of an inner sheet with an outer sheet, which requires access to opposite sides of the sheets to permit joining thereof by the welding electrodes, followed by a projection welding step of the inner sheet with another outer sheet.
FIG. 1 shows an example of a joined sheet metal stack 10, composed of a first outer sheet 12, an inner sheet 14, and a second outer sheet 16, in the form of a metal strap which is considerably thicker in relation to the first and second sheets. The second outer sheet 16 is joined to the inner sheet 14 by a resistance weld 18a, and the first outer sheet 12 is joined to singly-apexed projections 14a of the inner sheet 14 via projection welds 18b. Merely be way of an environment of utilization, FIG. 1A shows an assembly 10′ consisting of a roof panel 12′ serving as the first outer sheet, which is to be welded to a body panel 14′ serving as the inner sheet, and a strap spacer 16′ serving as the second outer sheet, which is to be welded to the body panel opposite the roof.
Details of the two-sided, two step joining method of the prior art will be discussed with reference additionally to FIGS. 2A through 2F.
At FIG. 2A, a die 20 having interfacing die components 20a, 20b, wherein complementing v-shaped die faces 20a′, 20b′ are diametrically opposed to each other, is utilized to apply stamping forces F1′ upon the inner sheet 14 to thereby form at least one pair of singly-apexed projections 14a, each being in the configuration of a general v-shape, in the inner sheet. By way of example, the resulting stamped inner sheet 14, as shown at FIGS. 2B through 2D, may have elongated singly-apexed projections 14a having a raised height H′ of about 0.7 mm, a width W′ of about 1.5 mm and a length L′ of about 8 mm.
The first welding step of the prior art is depicted at FIG. 2E. The second outer sheet 16 is brought into abutment with the inner sheet 14, wherein the singly-apexed projections 14a face away from the second outer sheet. Diametrically aligned electrodes 24a, 24b of a resistance welding apparatus 24 are bought into abutment with opposing sides of the second outer sheet 16 and the inner sheet 14 between the singly-apexed projections. With opposing forces F′2 being applied to the electrodes toward the first and second outer sheets, current is supplied to the electrodes, resulting in a resistance weld 18a. 
The second welding step of the prior art is depicted at FIG. 2F. The first outer sheet 12 is brought into abutment with the singly-apexed projections 14a of the inner sheet 14, which has been already welded to the second outer sheet 16 via the first welding step. Welding electrodes 28a, 28b, one for each singly-apexed projection 14a, of a projection welding apparatus 28 are brought into abutment with the first outer sheet with applied forces F3′ theretoward, wherein each electrode superposes a respective singly-apexed projection. Current i′ is supplied to the electrodes, wherein the current passes through the abutting interface of the first outer sheet and the singly-apexed projections into the second outer sheet, and passes through the first outer sheet 12 as a current i1′, through the inner sheet 14 as a current i2′, and through the second outer sheet as a current i3′ which, because the second outer sheet is much thicker than that of the first outer sheet and the inner sheet, it carries most of the current between the electrodes, resulting in the projection welds 18b. 
The result of the prior art first and second welding steps is shown at FIG. 1. Any number of sets of welds 18a, 18b may be provided as between the sheets 12, 14, 16 in the manner above recounted for each set of singly-apexed projections.
While the two-sided, two step method of joining three sheets to each other works well, it has a number of disadvantages, mainly because it inherently requires multiple steps, and opposing sheet sides must be accessible to the welding electrodes (at least for the resistance welding step). Under the prior art joining methodology, neither of the two welding steps is capable of simultaneously welding all three sheets; for example, the large surface contact between the second outer sheet and the inner sheet precludes the second welding step from accomplishing this feat.
Therefore, what remains needed in the art is a joining methodology for three metal sheets which may somehow be performed with only one sheet side accessibility, and, further, be somehow performed using only a single welding step.